1. Field of the Invention
The present invention relates to a radiation image detector for detecting a radiation image.
2. Description of the Related Art
A radiation imaging system such as X-ray imaging system is constituted of an X-ray generator for generating X-rays and an X-ray imaging device for capturing an X-ray image of a subject. The X-ray generator has an X-ray source for irradiating X-rays toward a subject, an X-ray source controller for controlling operation of the X-ray source, and an irradiation switch for entering a command for starting irradiation of X-rays. The X-ray imaging device has an X-ray image detector for detecting an X-ray image in response to the X-rays having transmitted through the subject, and an imaging controller for controlling operation of the X-ray image detector.
An X-ray image detector provided with a flat panel detector (FPD) instead of an X-ray film or an imaging plate (IP) as a detector has been in widespread use, recently. The FPD has a matrix of pixels for accumulating signal charges by an amount corresponding to an amount of the X-rays incident thereon. In the FPD, the signal charges are accumulated in each of the pixels, and a signal processing circuit converts the accumulated signal charges into a voltage signal (pixel signal), so as to detect an X-ray image representing image information of the subject and output the X-ray image as digital image data.
A portable X-ray image detector (hereinafter referred to as electronic cassette) in which a FPD is contained in a rectangular parallelepiped housing has been in practical use. The electronic cassette is attached to an imaging table designed for a film cassette or an imaging plate (IP) cassette, and used. Alternatively, the electronic cassette is held by a subject himself/herself, and used so as to capture an image of a body site which is difficult to capture by a floor-standing type electronic cassette. Further, in some cases, the electronic cassette is taken out from a hospital to somewhere having no imaging table and used therein, so as to capture an image of a housebound elderly person or an emergent patient suffered from an accident or a natural disaster.
Generally speaking, output of an electrical component is susceptible to noise caused by an internal factor of the electrical component itself or an external factor such as an ambient environment. Of course, the X-ray image detector equipped with a lot of electrical components is no exception. When the subject or radiographer happens to bump into the X-ray image detector and the X-ray image detector is applied with impact, the X-ray image detector vibrates in response to the impact, and the vibration of the X-ray image detector causes noise. When the noise is applied to a signal of image data, the noise causes a false image on the X-ray image, and the false image may lead to diagnosis error.
In view of the above, there have been conventionally proposed various methods for preventing degradation of image quality of an X-ray image due to noise. According to Japanese Patent Laid-Open Publication No. 2009-081443, in order to eliminate noise generated by potential fluctuation of a scanning line of a TFT provided in a FPD, there are provided dummy pixels each having no function of accumulating signal charges corresponding to the X-rays, and an output signal of the dummy pixels is subtracted from an output signal of effective pixels. According to Japanese Patent Laid-Open Publication No. 2009-128186, in order to eliminate noise generated by fluctuation of bias voltage entirely applied to a FPD, a filter circuit and a buffer circuit are connected between the FPD and a bias supply.
As the signal processing circuit for converting the signal charges accumulated in the pixels into the voltage signal, a dedicated integrated circuit such as application specific integrated circuit (ASIC) is used, for example. The ASIC is arranged for each group containing plural rows of pixels (256 rows of pixels, for example). There are individual differences among the ASICs, and therefore introduction of the noise varies in accordance with the group corresponding to each of the ASICs. Further, the ASIC itself is a generation source of the noise.
Since the introduction of the noise varies in accordance with the group corresponding to each of the ASICs as described above, even if the output signal of the dummy pixels is uniformly subtracted from the output signal of the effective pixels as disclosed in Japanese Patent Laid-Open Publication No. 2009-081443, it is not possible to eliminate the noise components sufficiently, or image components which are not the noise components are unnecessarily eliminated. Moreover, according to the method disclosed in Japanese Patent Laid-Open Publication No. 2009-128186, although it is possible to eliminate the noise components generated by the fluctuation of bias voltage, it is impossible to eliminate the noise components generated by the ASICs themselves.